In the manufacture of molded records, such as conventional audio records or the more recently developed video discs, electroforming is used in a series of critical process steps. In the manufacture of video discs, for example, the program information is cut into a metal recording substrate which is made by electroforming a relatively thick layer of metal, such as bright acid copper, on a rigid support. After the desired program information has been cut into the recording substrate the substrate is replicated by electroforming a metal on the surface of the recording substrate to produce a matrix part referred to as a master. The master in turn is replicated by electroforming a metal on its surface to form on or more matrix parts which are referred to as molds. The molds in turn are replicated by electroforming a metal, typically nickel, on the surface of the molds to produce matrix parts referred to as stampers. The stampers, if properly made, will be exact negative replicas of the recorded substrate. The stampers are the matrix parts which are actually used to mold the plastic records.
If the series of electroforming replication steps noted above are carried out correctly, the audio records or the video discs which are pressed with the stampers will, on playback, produce a high fidelity reproduction of the program information which was cut into the recording substrate. If has been found, however, that in the electroforming process defects are often introduced into the replicas. The defects are typically small pits or voids but also include embedded particles. The formation of defects in matrix parts during electroforming is additive in nature in that the defects introduced in each electroforming step are replicated in each subsequent part. For example, defects in the recording substrate are reproduced in the master, mold, stampers and finally in the molded records pressed with the stampers. In a like manner, defects formed in the master are added to the defects from the recording substrate and reproduced in the molds and so forth.
The defects which occur in the matrixing process are typically relatively small by conventional standards, being on the order of a micron to a few microns in size and accordingly are called "microdefects". Microdefects, despite their name, are a major concern in the manufacture of records in general and particularly in the manufacture of video discs. In a video disc the signal tracks are only about 4,000 angstroms deep and the signal elements are only about 500 to 1000 angstroms in amplitude. Furthermore, the signal information tracks are only about 2.7 microns wide and are spaced about 4,000 per centimeter across the recorded surface of the video disc. Therefore, a microdefect in an electroformed part for video disc molding which is a few microns in diameter or only a micron or so in depth can constitute a major defect in the video disc because of the relatively small size of the topographical features molded into the discs. Microdefects can cause the recorded program information to be grossly distorted or even cause a complete loss of picture and sound on playback of a video disc. Further common problems caused by microdefects includes "locked groove" wherein the video discs continue to repeat a particular segment of the recorded program and "skipped grooves" wherein the stylus of the video disc player skips over a series of grooves, causing discontinuities in the recorded program information. The microdefects can also cause excessive wear and even breakage of the diamond styli used in the video disc players.
The microdefects which are produced in the electroformed parts are to a large extent caused by foreign materials in the electrolyte used in the electroforming process. The foreign materials are introduced in the electrolyte from many different sources such as from the metal supplied to the anodes, from the parts that are to be electroformed, produced by chemical breakdown of the electroforming solutions and so forth. The foreign materials found in the electrolyte can be present as liquids or solids. Solid particles if they are present can become embedded into the electroformed matrix part, causing either a hard spot or a soft spot in the final part, both of which cause substantial problems especially in the recording substrates. Liquids such as oils and the like can effectively prevent metal deposition in a given area, causing a void or pit to form in the electroformed parts which results in a pit or bump in the molded record, depending upon the particular step in the matrixing process where the defect occurs.
Various methods have heretofore been used to remove foreign materials from the electroforming solutions. One such method is to circulate and filter the solution during electroforming to remove foreign materials. However, one of the problems which has been encountered in circulating and filtering of the electrolyte is that in the conventional electroforming tanks, heretofore used in matrixing of parts for record part, the foreign materials, instead of being removed from the tank for filtering, tend to accumulate in the electrolyte and particularly to accumulate immediately adjacent to the part being replicated. The accumulation of foreign materials immediately adjacent to the part being electroformed is partly caused by poor circulation and because the flow patterns in many of the prior art electroforming tanks causes the revolving cathode head acting as a dam against which the foreign particles tend to collect. It has been suggested in the prior art to use a stand pipe or the like to remove the electrolyte for the purposes of filtering it, but this at best has only a localized effect and does not remove the accumulated foreign materials from the surface of the cathode. It has also been suggested to use bottom drains to remove the electrolyte, but this allows floating foreign materials to accumulate on the surface of the electrolyte bath which continue to cause problems in the electroforming process.
A highly advantageous electroforming apparatus for use in the matrixing of electroformed parts for record manufacture would have characteristics that would eliminate or substantially reduce the formation of microdefects in the electroformed matrix parts.